1) To hinder or prevent the passage of. → stopping power.
2) The diaphragm used in optical instruments to cut off the marginal portions of a
beam of light passing through lenses. → field stop;
→ stop number.

A quantity indicating the extent with which a substance absorbs a
→ charged particle passing through it. It is
the energy lost by a → non-relativistic particle per unit
length of its path in the substance.